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Configuring global CAR
Overview
Global committed access rate (CAR) is an approach to policing traffic flows globally. It adds flexibility
to common CAR where traffic policing is performed only on a per-traffic class or per-interface basis.
In this approach, CAR actions are created in system view and each can be used to police multiple
traffic flows as a whole.
Global CAR provides the following CAR actions: aggregate CAR and hierarchical CAR.
Aggregate CAR
An aggregate CAR action is created globally. It can be directly applied to interfaces or used in the
traffic behaviors associated with different traffic classes to police multiple traffic flows as a whole.
The total rate of the traffic flows must conform to the traffic policing specifications set in the
aggregate CAR action.
Hierarchical CAR
A hierarchical CAR action is created globally. It must be used in conjunction with a common CAR or
aggregate CAR action. With a hierarchical CAR action, you can limit the total traffic of multiple traffic
classes.
A hierarchical CAR action can be used in the common or aggregate CAR action for a traffic class in
either AND mode or OR mode.
• In AND mode, the rate of the traffic class is strictly limited under the common or aggregate CAR.
This mode applies to flows that must be strictly rate limited.
• In OR mode, the traffic class can use idle bandwidth of other traffic classes associated with the
hierarchical CAR. This mode applies to high priority, bursty traffic like video.
By using the two modes appropriately, you can improve bandwidth efficiency.
For example, suppose two flows exist: a low priority data flow and a high priority, bursty video flow.
Their total traffic rate cannot exceed 4096 kbps and the video flow must be assured of at least 2048
kbps bandwidth. You can perform the following tasks:
• Configure common CAR actions to set the traffic rate to 2048 kbps for the two flows.
• Configure a hierarchical CAR action to limit their total traffic rate to 4096 kbps.
• Use the action in AND mode in the common CAR action for the data flow, and in OR mode in the
common CAR action for the video flow.
The video flow is assured of 2048 kbps bandwidth and can use idle bandwidth of the data flow.
In a bandwidth oversubscription scenario, the uplink port bandwidth is lower than the total downlink
port traffic rate. You can use hierarchical CAR to meet the following requirements:
• Limit the total rate of the downlink port traffic.
• Allow each downlink port to forward traffic at the maximum rate when the other ports are idle.
For example, you can perform the following tasks:
• Use common CAR actions to limit the rates of both Internet access flow 1 and flow 2 to 128
kbps.
• Use a hierarchical CAR action to limit their total traffic rate to 192 kbps.
• Use the hierarchical CAR action for both flow 1 and flow 2 in AND mode.
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